The effect of interstitial fluid on the machining behaviour of cortical bone

Bone machining research is usually conducted ex-vivo with standard laboratory equipment, implying mostly that the cutting process takes place in dry state. While these studies are useful for understanding the bone cutting process, they cannot represent the real clinical cutting condition due to the disregard of internal irrigation (i.e., interstitial fluid and blood within the tissue's vascular porosities). As bone possesses ca. 20 % water, internal irrigation can influence the bone's properties (e.g., shear strength, friction coefficient) and alter the cutting mechanism and surface deformation (e.g., crack formation, smearing). Hence, to study the ex-vivo bone cutting process in an internally irrigated state, a novel machining and pumping setup is proposed here, which enables a scenario closer to a surgical condition than the traditional dry machining method. Cutting tests demonstrated that portions of bone near the fluid-filled porosities possess shear strength and friction coefficient gradients due to the bone's permeability. This resulted in machining forces and necrosis up to 52 % and 55 % lower than in dry bone, respectively. A cutting forces model was developed considering the internal irrigation condition and the randomness and anisotropy of the tissue. Moreover, it was found that internal irrigation can change the chip formation mechanism and induce minimised surface morphological damage due to the minimised friction between the tool and the cutting surface in near-porosity sites. This research highlights the importance of conducting bone machining research with the consideration of internal irrigation, an imperative aspect to more realistically mimic surgical scenarios in a laboratory environment.